A radio receiver is designed to be extremely sensitive to responding to the desired signals to which it is tuned and to rejecting signals to which it is not tuned. Superheterodyne radio receivers typically achieve the bulk of their selectivity using narrow fixed-tuned intermediate frequency (If) filters. These filters reject undesired signals which may be present at RF frequencies which are close to the desired or "tuned" frequency. Such narrow If filters do not prevent the reception of undesired signals which are converted in a mixer directly into the If passband. Such undesired signals which are not rejected by the radio receiver filters create spurious responses such as image, half If and intermodulation (intermod) distortion.
These spurious responses may be generated in part due to the operation of the mixer in which various mixing products are generated, one of which is at the desired If frequency. One approach to eliminating such spurious responses is to use bandpass filters to prevent undesired signals from reaching the mixer in the superheterodyne radio receiver.
In the past, it was common practice to use a very narrow RF bandpass filter to prevent such undesired signals from reaching the mixer. Such "front end selectivity" was effective at eliminating most spurious responses, but the resulting receiver could only be operated over a very narrow band of desired receive frequencies without retuning the narrow RF filter. Thus, although narrow RF bandpass filters are effective in eliminating, for example, image and half If spurious responses, such prior art filters are limited to receiving a narrow band of frequencies and cannot be reconfigured automatically to receive signals over a wide frequency band.
Many modern day receivers utilize a much wider front end filtering system designed to pass the entire band of frequencies to which the receiver may be tuned (e.g., 150-174 MHz for a particular band used in the land mobile radio service). By careful design and proper selection of the If frequency, such a wideband filter can be designed to provide adequate protection from image and half If spurious responses. However, a wideband front end filtering system greatly increases the incidence of interpod distortion in the receiver by allowing a much larger number of in-band signals to reach the mixer.
Such an intermod spurious response is created when two or more undesired signals reach a non-linear element in the receiver such as the first mixer and combine to create a product that is directly on channel. Accordingly, such an on-channel signal cannot be readily distinguished as an undesired signal. Such undesired signals, which are mixed to produce an on-channel signal, may be so close to being on-channel themselves that they are difficult to reject with a wideband filter.
Various filter designs have been used heretofore to provide an automatically tuned front-end filter. Each such design, however, has significant disadvantages, particularly, for example, in regard to satisfying the needs of a mobile or portable radio communications system.
In broadcast receivers, a mechanically variable capacitor having two or more sections which are "ganged together" is used to tune the local oscillator and front end filter simultaneously. While such an approach may be acceptable in an AM broadcast radio receiver, where the bandwidths are very wide, this approach is not suitable for mobile radio receivers which have extremely narrow channel spacing and which cannot tolerate the poor frequency stability of such a mechanically tuned local oscillator. Thus, in a mobile radio receiver, if the filter detunes or the local oscillator drifts by any small amount due to such poor frequency stability, desired signals may not be received.
Another approach which heretofore has been utilized to provide an automatically tunable front end filter, uses multiple front end filters, each of which is tuned to a different frequency within the overall passband. Such a system includes control circuitry in the receiver which selects the appropriate filter based on the radio channel selected and switches it to the front end via, for example, a relay or a PIN diode switch. Each of the multiple front end filters is typically a narrow fixed tuned filter. In order to effectively cover a wide band of frequencies using such narrow filters, a large number of filters is required. Accordingly, such a design from a practical point of view is prohibitively costly, complex and bulky.
A further approach to providing an automatically tuned front end filter incorporates voltage variable capacitors (i.e., varactor diodes) as reactive elements in a single filter. A control voltage is applied to the varactor diodes to tune the filter to the desired receive frequency. This approach has heretofore typically utilized a reactively coupled resonator filter which generally has multiple sections that are tuned separately but which must "track" as the tuning voltage varies. Such a multiple section filter must have sections which have the same tuning response so that each section tunes in exactly the same way when the tuning voltage varies. To ensure that such "tracking" takes place in the proper fashion adds to the complexity of the filter. Additionally, although such a filter may be effective to reduce image and half If spurious responses, in such prior art tunable filters which incorporate varactor diodes, the diodes tend to be subjected to high RF levels at all frequencies and have the potential to generate intermod products themselves.
The present invention overcomes many of the above-mentioned problems associated with conventional radio receiver filters in superheterodyne radio receiver applications. The present invention provides a narrow front end filter which can be tuned automatically as the local oscillator frequency is changed. In this fashion, the present invention reduces the number of spurious responses in the radio receiver while still allowing it to tune automatically over the full range of the desired frequency band.
In this regard, it is noted that the tuning voltage is used to control a single point in the circuit of the exemplary embodiment of the present invention. The tuning voltage is input at the junction between two varactor diodes which are placed back to back to thereby provide more linear tuning of center frequency versus voltage while providing higher intermod immunity than would be available using a single varactor diode. Additionally, the need for "tracking" between multiple diode sections is avoided. In the present invention, the varactor diodes are significantly decoupled from high RF levels. This results in less voltage being coupled across the varactors, reducing the possibility of generating unwanted intermod products.
The electrically tuned bandpass filter of the present invention uses the signal isolating properties of a balanced resistive bridge to transform a series tuned reflective notch filter into a selective bandpass filter. While any directive device such as a directional coupler, ferrite circulator, Wilkinson splitter, etc., could be used to provide this transformation, the resistive bridge of the exemplary embodiment additionally operates to provide an apparent "Q multiplication" in the tuned circuit yielding a sharper selectivity curve.
In the present exemplary embodiment, the series tuned reflective notch filter is coupled to a node of the balanced resistive bridge in such a manner that the tuned circuit presents a short circuit to the node at the filter's resonant frequency to thereby assure a maximum transfer of signal from the input to the output of the circuit at a desired frequency. At all other frequencies, however, the series tuned reflective notch filter will present essentially an open circuit across the node of the resistive bridge so that the node is balanced and no transmission occurs. The inclusion of the series reflective notch filter thereby produces a bandpass characteristic from the circuit input to output.
By using varactor diodes in the reflective notch filter, the center frequency of the passband can be changed by changing the tuning voltage applied to the varactors. In the configuration of the present invention, the varactor diodes are protected from high signal levels which are not within the narrow passband of the filter. For this reason, the filter is inherently less susceptible to the generation of intermod products than filters of a conventional design.